Roots pumps typically comprise a pair of meshed, lobed rotors which rotate within a housing, causing fluid to become trapped in pockets surrounding the lobes and to be transferred from the pump inlet to the pump outlet. The rotors do not actually touch each other or the housing, so no lubricant is needed. This makes Roots pumps desirable in applications where contamination of the fluid is a problem, for example in semiconductor processing.
A simplified diagram of a typical Roots pump 100 is shown in FIG. 1. A pumping chamber 101 is formed by a plurality of stator components, including a stator housing 102 and two transverse end walls 104. The end walls 104 have apertures 106 through which two rotor shafts 108, 110 extend. The shafts are supported at each end by bearings 112. A motor 114 drives rotation of one shaft 108 and a gear mechanism 116 transmits the rotational power to the other shaft 110. The gear mechanism causes the shafts to rotate in synchronisation in opposite directions.
The shafts have mounted thereto respective pairs of rotor lobes 118, 120 and 122, 124. The radial tip of lobe 122 is hidden by lobe 120 and therefore is designated by broken lines. FIG. 2 shows a section through pump taken along the line II-II, in which the rotor lobes can be seen more clearly. As the rotors rotate, the lobes sweep past the internal surface 126 of the pumping chamber 101 thereby pumping fluid from a chamber inlet 128 to a chamber outlet 130. The tolerances between the rotor lobes and the swept surface 126 must be tightly controlled, as must the tolerances between the rotors, otherwise gaps will be generated through which fluid can pass, thereby decreasing the efficiency of the pump. Typical tolerances are in the region of 0.1 mm.
Typical Roots pumps have a reasonably high pumping capacity, but for some applications it is desirable to further increase the capacity of the pump. This can be achieved, whilst maintaining lobe tip speed, by providing a larger pump with bigger lobes. However, this is disadvantageous in that the pumps become more expensive, and if there is an accident, for example if the rotors clash, the increased energy of the lobes can be sufficient for the lobes to break through the pump housing and cause damage or injury.
Alternatively, the capacity of the pump can be increased by causing the rotors to spin faster. A typical lobe tip speed during rotation is less than 100 m/s and often less than 80 m/s. A significant increase in velocity at the tip of the lobes to for example 130 m/s would allow the lobes to be made smaller, and reduce the cost of the pump. However, even though the lobes are less massive, the increased rotational speed causes an increase in lobe energy, and in the event of an accident can likewise cause damage or injury. It should also be noted that increasing the speed causes a larger increase in kinetic energy than increasing the mass, since the energy is proportional to the mass but it is proportional to the square of the speed.
Conventional rotors are usually made from a solid block of material, typically cast iron. Such rotors may be made in various ways, including casting solid lobes and a shaft integrally, or casting solid lobes and attaching the lobes to a shaft to form the rotor.
Known lobes may be manufactured by casting a solid lobe and then drilling a hole in it to reduce its weight.